1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to architectures for multimedia including data/voice services to mobile users using stratospheric platforms.
2. Description of the Related Art
Stratospheric platforms are being considered for multimedia including data/voice communication applications. Current proposals envision a mounting of transceivers and antennas on aircraft flying at 20-30 kilometers above the earth which will project beams to cell sites located on the ground.
Conventionally, cells projected on the ground are uniform in size. If the user distribution is uniform, the equal sized cell structure is optimal. However, equal cell size comes at some cost in hardware. To avoid mechanical tracking systems on the antennas, which can be costly and unreliable, the antennas are typically implemented as phased arrays of radiating elements and steered electronically. At the altitude above the ground, where the payload carrying platform is located, a same-sized ground projection of cells requires smaller angular beams as the scan angle increases. To form smaller beams, more antenna array elements are needed. For a light-weight payload, the number of elements may be limited, thus forming smaller beams at the edge of the coverage may be costly.
Further, prior stratospheric based platform proposals envision a fixed cell structure by which the beams stare at all cells in the entire coverage area, similar to a cellular system. If resources are available, this approach is viable. In some systems, however, resources may be limited. For such systems, use of a fixed cell structure limits the coverage area. Consequently, total system capacity is reduced. That is, at any given time, the entire coverage area may not be covered by beams that provide adequate link margin for billable data transmissions.
Hence, there is a need in the art for a stratospheric platform based communication system offering maximum throughput with the constraints of weight, power and spectrum. More specifically, there is a need in the art for a stratospheric platform based communication system and method for projecting beams of varying cell structure in both time and space to maximize system capacity within the weight, power and bandwidth constraints thereof and thereby to optimize the communications capacity for billable voice and data transmissions.
The need in the art is addressed by the communication system of the present invention. The inventive communication system includes a first transceiver located on a first platform at a predetermined altitude. A first antenna ( array ) at, say, S-band is located on the first platform and connected to the first transceiver. A second (high gain) antenna at, say, X or C-band is connected to the other end of the first transceiver. A second transceiver is located on a ground hub physically and is independent of the first platform. A third antenna at, say, X or C-band, is located on the ground hub and connected to the second transceiver. The third antenna is adapted to communicate with the first platform via a second ( high gain ) antenna at X or C-band on the first platform. A beamforming system is connected to the second transceiver and mounted on the ground hub. The beamforming system provides beamformed signals from the second transceiver to the first transceiver effective to drive the first antenna array to radiate multiple beams to a surface, whereby the multiple beams create time varying and dissimilar footprints thereon.
In the illustrative embodiment, the first platform is maintained in a stratospheric orbit, the second transponder is located on the ground hub. A second (high gain) antenna is mounted on the first platform to receive the beamformed signal from the ground hub in the forward link direction. The beamforming system on ground is adapted to drive the first array antenna on the first platform to generate plural beams on the earth""s surface, each beam providing a respective footprint or cell. Similarly, the multiple user signals, arrived at the first antenna array in the return link direction, will be amplified, filtered, frequency translated, code multiplexed, amplified again, and then radiated through the second (high gain ) antenna to the ground hub. The third antenna on the ground hub will receive the multiplexed element signals, which will then be amplified, filtered, down-converted, and demodulated to recover individual clement level signals before digitization. The digitized element level signals will be pushed through the digital beam forming network, which separates user signals via their angle of arrivals (with respect to the first array antenna on the platform).
Each beam tracks a respective user located at a center of each cell. The system allows for narrow beams to be created which, in turn, enables frequency reuse. A code is assigned to each beam and a mechanism is provided for preventing a user from receiving more than one beam with a given code. This mechanism is adapted to anticipate a condition by which a user may move to a location at which the user would receive more than one beam with a given code. It will assign a second code to at least one beam prior to the arrival of the user at that position.
The present invention allows the cell size to be non-uniform. That is, near center of coverage, or nadir, the cell can be smaller. As the scan angle increases, the cell sizes increase. Cell size equalization may require additional apertures or much more array elements. It will drive the payload weight and cost significantly. The invention allows for a light-weight payload design and full utilization of the resources that a light-weight payload can offer.
The present invention forms beams where there are users present with beams of shapes and sizes that are not necessarily uniform in space and constant in time. One or more broad beams may be formed to provide links supporting lower data rates. These lower data rate links are used to support acquisition protocol for new users trying to get on the system. This allows the coverage area to be greater with limited receiving beams. In addition, by allowing the beam size to be smaller near the center of coverage (nadir of the platform), the code or frequency reuse distance can be reduced, therefore enhancing the total system capacity.